1. Field of the Invention
The present invention relates to a damper having a mechanism for restraining the propagation (conduction) of vibrations produced in a vacuum pump.
2. Description of the Related Art
As an apparatus using a vacuum stem such that the interior thereof is kept in a vacuum by accomplishing evacuation using a vacuum pump, semiconductor manufacturing equipment, an electron microscope, a surface analyzer, fine processing equipment, and the like can be cited.
Also, of various types of vacuum pumps, a vacuum pump that is often used to realize a high-vacuum environment is a turbo-molecular pump.
The turbo-molecular pump is configured so that a rotor rotates at a high speed in a casing having a suction port and an exhaust port. On the inner peripheral surface of the casing, stator blades are disposed in a multistage manner. On the other hand, on the rotor, rotor blades are disposed radially in a multistage manner. When the rotor is rotated at a high speed, gas is sucked through the suction port by the action of the rotor blades and the stator blades, and is discharged through the exhaust port.
In the turbo-molecular pump, when the rotor is rotated at a high speed, vibrations are produced by the cogging torque of motor. Also, in the case where the rotor is not balanced completely, vibrations caused by the oscillation of shaft may also be produced.
If such vibrations produced in the vacuum pump propagate to the vacuum system side, the performance of the system may be hindered. For example, the electron microscope and the surface analyzer may be greatly affected even by minute vibrations on the micron order or the nano order.
To solve this problem, a damper is arranged between a vacuum chamber and the vacuum pump to keep vibrations (including minute vibrations) produced in the vacuum pump from propagating to the vacuum chamber.
As the damper, for example, a thin and stainless steel made member (bellows) the peripheral surface of which is bent in a bellows form, the member being coated with an elastic material such as silicone rubber, is used. When the vacuum pump is in use, a hose band etc. are mounted on the damper from the outside to tighten the damper.
The damper is preferably set so that the natural frequency of the whole of the damper system is low (for example, 100 Hz or lower) to increase the damping characteristics, that is, to improve the vibration isolating properties.
The natural frequency can be reduced by decreasing the rigidity and damping coefficient of damper.
However, since the damper is arranged in the air, a structure capable of withstanding a difference in pressure between a vacuum pumping flow path and the atmosphere is necessary. Therefore, the damper requires a certain degree of high rigidity. That is to say, there is a limit in decreasing the rigidity of damper.
At the time of acceleration or deceleration when the vacuum pump is started or stopped, the number of revolutions of the rotor passes through the natural frequency of the whole of the damper system. At this time, namely, when the frequency of rotor and the natural frequency of the whole of the damper system coincide with each other, resonance occurs.
Also, in some cases, vibrations of this frequency are excited by a force supporting the rotor or disturbance transmitted from a pipe etc. In particular, on a band near the natural frequency, vibrations sometimes increase remarkably as compared with the case where the damper is not used.
The resonance phenomenon produces large vibrations even under a low force. However, the displacement (vibration amplitude) at the time of resonance can be decreased by increasing the damping coefficient of the whole of the damper system.
To reduce the influence exerted by such resonance, the damper is required to have a certain degree of high damping coefficient. That is to say, there is also a limit in decreasing the damping coefficient of damper.
Accordingly, various techniques for reducing the natural frequency of the whole of the damper system have conventionally been proposed, including patent documents described below.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 59-221482    Patent Document 1: Japanese Utility Model Registration No. 3092699
Patent Document 1 proposes a technique in which a balancing vacuum chamber is provided to cancel a suction force that the vacuum pump is subjected to. By providing such a balancing vacuum chamber, a compressive force acting on the bellows can be restrained. Thereby, the bellows can be caused to function with a substantially free length even if the bellows is formed by a softer member, so that the natural frequency of the whole of the damper system can be reduced.
Also, Patent Document 2 proposes a technique in which the force of evacuation is received by a stranded wire. By the configuration capable of withstanding the vacuum force using the stranded wire, the natural frequency of the damper can be reduced.
However, in the case where the natural frequency of the whole of the damper system is reduced by decreasing the damping coefficient, the stability of vacuum pump at the time when the vacuum pump is subjected to disturbance may be impaired.
Also, in the case where the natural frequency of the whole of the damper system is reduced by decreasing the rigidity of damper, the vacuum pump may become incapable of being supported stably when the vacuum pump is installed in an upside down posture (the suction port is directed downward) or in a horizontal posture (directed horizontally). Specifically, in the case where the vacuum pump is installed in an upside down posture or in a horizontal posture, deflection or tilt may be produced by the own weight of vacuum pump. If a trouble such as deflection or tilt occurs, the degree of freedom (flexibility) of installation posture of vacuum pump is restricted.